IPv6 header insertion in a controlled domain

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Mark,

[...]
[renamed thread]

> Take an example of 3 routers A, B and C that is under the same control.
> Traffic travels from left to right.
> 
> A -- B -- C
> 
> A has a tunnel to C using ULA source and destination.
> A packet ingresses on A, is encapsulated and forwarded towards C.
> A has for reasons unknown to us outsourced a function to B. Where B insert an extension header in the packet originated by A.
> C is also in on the game, being a tunnel end-point, removing the encapsulating header and extension header.
> 
> If you can leave the nuances of SR and the whatever they propose on the shelf for a minute.
> 
> Do you agree in principle that this approach of "header insertion in a controlled domain" can be made to work,
> and should be indiscernible from A inserting the extension header itself?

Yes, there are obviously many ways of skinning this cat.
In this thread I don't want to try solve whatever problem the EH insertion camp wants solved. I don't think I understand it well enough.
And I'm unconvinced the 6man group in general understands the underlaying routing problems leading to the proposed solution.
What I was interested in exploring in this thread was if we could reach agreement that there in principle is a safe way of doing header insertion.

In my simple example above.
Do you agree that in a controlled distributed system there is no perceivable difference between node A inserting the header or node B doing it?
Conceptually you could think of it as a tunnel with muliple endpoints (although that's a bit hard to visualize ;-)).

Can we agree that inserting headers and changing the packet size among participating nodes in a distributed system is not inherently unsafe?
As far as I can think of the only complication to be considered is that node A must be aware of packet size changes when doing PMTU estimation.
(typically though in these types of networks, one simply declares that "MTU must be well managed. i.e. large enough".

Cheers,
Ole